1. Field of the Invention
The present invention generally relates to image forming methods and image forming apparatuses. More particularly, the invention relates to the reduction of a banding problem in an overlap region in a printed medium.
2. Description of the Related Art
There are various image forming apparatuses, such as printers, facsimile machines, copiers, and multifunction peripherals (MFP). An inkjet printer is one type of image forming apparatus that typically has a recording head configured to discharge a droplet of recording fluid (such as ink) onto a print medium, such as a sheet of paper, thereby forming an image on the print medium. The “print medium” is not limited to any particular material and may be herein referred to as a “sheet”, a “recorded medium”, “print medium”, “transfer material”, or “recording paper”. To “form” an image is synonymous with “recording”, “printing”, “transferring”, or “transcribing” an image by causing a recording fluid to attach to a print medium.
In an inkjet printer, the recording head may have multiple nozzles for discharging ink droplets in order to realize a high-speed printing (i.e., image-forming) operation. In a serial-type inkjet printer, ink droplets are discharged while the recording head is moved in a direction (“main-scan direction”) perpendicular to a direction in which the print medium is transported (“sub-scan direction”). In order to achieve high image quality, a multi-pass method may be adopted, whereby the same region in the sheet is scanned in a main-scan direction more than once (“multiple scan”) by the same group of nozzles or different groups of nozzles. However, the multi-pass method is subject to limited print speed due to its need to scan in two directions, i.e., the main-scan direction and the sub-scan direction. Thus, there is a need for increasing printing speed of an inkjet printer.
The printing speed of an inkjet printer may be increased by moving the recording head in only one direction (such as in the sheet-feeding direction). However, this requires a recording head having a width greater than the width of the sheet. Desirably, such a wide recording head should have the nozzles arranged in a row at certain intervals (“line type”). There is, however, a limit to the number of nozzles that can be provided to the recording head due to cost and manufacturing technology reasons. Thus, in many cases, a plurality of recording heads are used in combination, each recording head having a certain number of nozzles.
One problem associated with this solution is that those multiple recording heads can be joined together with only limited accuracy, and the nozzle-to-nozzle distance between adjacent recording heads cannot be made completely uniform. As a result, if the nozzle-to-nozzle distance is too large, a white line may be caused by a reduced amount of ink, while a dark line may be caused by excessive amount of ink if the nozzle-to-nozzle distance is too small. The appearance of such lines in the printed medium is referred to as “banding”.
In order to prevent such banding phenomenon, an overlap may be provided at the joint of adjacent recording heads. An “overlap region” in the present disclosure refers to a region where more than one nozzle is assigned for generating a single dot. The overlap region may include a region that is scanned in the main-scan direction multiple times by a single recording head, or a region formed at a head-joint portion of a plurality of recording head members. However, in such an overlap region, because there are two or more nozzles corresponding to a dot according to data, the dot is produced by multiple nozzles unless some control is exerted, resulting in the development of banding due to increased density in the overlap region.
Japanese Laid-Open Patent Application No. 2005-41008 (“Patent Document 1”), for example, discloses a recording apparatus in which the sheet feed amount is varied when printing adjacent dots in the same main-scan line in the overlap region through different main-scan passes. However, the technology according to Patent Document 1 does not specify in which sub-scan direction the dots in the same main-scan line in the overlap region are shifted, and cannot prevent banding sufficiently.